Cameron on Saturday brought forward the launch of "Help to Buy" aimed at helping people who have been frozen out of the property market, a surprise move which defied expectations that the scheme could be watered down amid risks of a housing bubble.

The announcement added 310 million pounds to the market value of the four companies.

Help to Buy was originally launched to help buyers of new properties and the second phase, originally due to start in January, was designed to assist buyers who might otherwise be unable to afford a down payment on a home.

The latest phase will now start on October7, a government source said.

Analysts at Jefferies said the Prime Minister's move to bring forward the scheme was positive, particularly given their earlier expectations that "Help to Buy" could be scaled back, something which had been weighing on share prices.

"We view this news as a positive catalyst for all companies with exposure to UK housing transactions and would expect share prices in the UK homebuilding and estate agency sectors to make up the ground they have recently lost," they said.

Liberum capital analyst Charlie Campbell said it was significant that the scheme was now being championed by the prime minister as well as the chancellor, George Osborne, who had originally launched the idea.

"It means that the policy's become much more important to the government...therefore the chance of them pulling it in whatever way is much smaller. That's helpful to the whole industry," he said.

The sector was helped further by positive data on British mortgage approvals earlier on Monday, which hit their highest level in more than five years last month, and a sector update by JP Morgan.